Vaporization of a dense spherical cloud of droplets at subcritical and supercritical conditions

Droplet vaporization in a dense spherical cloud is studied numerically at subcritical and supercritical ambient conditions. The interaction among movable droplets is accounted for by adopting the concept of the variable-sized sphere of influence. Transient conservation equations for both the outer homogeneous gas-phase flow region and the inner heterogeneous two-phase flow region of the cloud are solved with high-pressure effects being taken into account, including ambient gas solubility, property variation, thermodynamic non-ideality, and transient diffusion. Effects of ambient pressure, droplet size, and cloud size on the fuel mass ejected from the cloud are investigated. At high pressures, due to the low liquid-to-gas density ratio, the fuel mass ejection from the cloud is much less than that at low pressures. Since this pressure effect is found to be much more important than the influences of droplet and cloud sizes, the external group combustion mode is less likely for a cloud of droplets at high pressures than that at low pressures with an equivalent spray density. Furthermore, since the droplet heating process in the cloud is entirely transient at high pressures, the conventional quasi-steady analyses that assume uniform droplet temperature are inadequate for the high-pressure vaporization of a dense cloud of droplets.